In general, the present invention involves agglomeration of finely divided material and more specifically relates to methods for wet pelletizing finely divided minerals and like substances into pellets of an advantageous pellet size distribution which achieves optimum bulk density with free flowing characteristics for shipping, packaging, and storing and reduces the amount of fines present while maintaining high dispersibility of the product.
Many powdery materials such as degritted and delaminated dry clay are dusty, non free-flowing and have a low bulk density of around 25 pounds per cubic foot. In order to induce such materials to flow from tanks, hoppers and chutes and other containers, vibrators, air pads, or hammers have to be used in order to prevent the fluffy material from caking or bridging. A finely divided material such as refined clay is very dusty in the dry state and, even with the best of air-tight containers and shipping vessels, much loss of the product and contamination of the surrounding environment occurs.
Apparatus revealed by the prior art for agglomerating finely divided powders include pin mixer type pelletizers and rotating tilted pan pelletizers. Materials pelletized in these devices achieve a fairly good compaction but produce pellets which are friable and substantially uniform in size. Also these devices produce fines and dust along with the pellets so that the dust problem is only reduced and not eliminated. The pellets are denser than the powdery material but not of the optimum density for shipping and dispersion.
The prior art has suffered from the difficulty of obtaining a pellet size distribution which allows the pellets to be placed in containers and storage vessels in the most compact manner. For example, pour densities of kaolin clays pelletized by methods of the prior art such as those disclosed by U.S. Pat. Nos. 3,446,218, 3,542,534, 3,460,195 and 2,758,039, usually range from 50 to 55 pounds per cubic foot, whereas the kaolin clays pelleted by the present invention exhibit pour densities in the range of 60 to 70 pounds per cubic foot.
Other deficiencies of the prior art processes include the high rate of attrition among the individual pellets in their containers which results in a large amount of undesirable fines. In order to reduce the attrition rate and prevent material loss and plant contamination due to excessive fines content, some of the prior art devices are designed to produce a hard pellet. While in some instances this serves to reduce the loss to fines, it always results in a pellet which is difficult to fracture and disperse into the final product such as paint, paper, ink, rubber, etc.
Also, the prior art processes include methods which call for adding all of the moisture of pelletizing to the substance before it is introduced into the pelletizer. This procedure even further reduces the flowability of the material and necessitates special conveying equipment such as screw conveyors to move the material from the wetting chamber to the pelletizer. Since the wetted material, having from 5 to 20% or higher moisture level, is especially susceptible to caking in the moist state, it is difficult to convey smoothly.
Other existing devices add all of the water of pelletization to the material while it is in the pelletizer. This results in a dry powdery product being introduced into the pelletizer which causes a large amount of dust in and around the pelletizer arising from the powdery material becoming airborne. Also the completely dry material in the pelletizer is more difficult to wet evenly than would be a slightly damp material just as a dry sponge is less absorbent than a slightly damp one.